This invention relates to a copying machine having an image density control device, in which the density of a copy image is controlled by detecting the density of an original to be copied.
To begin with, an electrophotographic copying operation is described according to FIG. 1 which shows a general type electrophotographic copying machine, schematically.
In FIG. 1, an original document (not shown) to be copied is placed on a copy board 1 such as glass and is pressed by platen cover 2. When a copy start button (not shown) is operated, an exposure light source 3 is turned on to start scanning in the direction of an arrow A. An image of the original is transmitted to a photosensitive drum 5 which serves as an electrostatic latent image carrier through an optical system 4 comprising a movable first mirror unit 41 having the exposure light source 3, a second mirror unit 42 movable synchronously with the first mirror unit 41, a fixed lens unit 43 and a fixed mirror 44, as per se well known.
The photosensitive drum 5 comprises an electrically grounded metal cylinder having on its outer peripheral surface a photoconductive layer such as selenium or the like, and it rotates in the direction of an arrow synchronously and in a link motion with the exposure scanning motion, of the exposure light source 3 described above. On the photosensitive drum 5, an electrostatic latent image of the original is formed in such a manner that the above-mentioned photoconductive layer is uniformly charged into positive electricity, for example, by a charge electrode 6 which is applied with a DC (Direct Current) high voltage of 5KV; photosensitive drum 5 receives a light image corresponding to the original thereon as optical system 4 is exposure-scanning the original; at this moment the conductivity of a part of the drum exposed to light is increased; the charge in this part escapes to the metal cylinder of the drum; positive charge remains in the dark parts; and thus, an electrostatic latent image corresponding to the original image is formed on the photoconductive layer.
When photosensitive drum 5 further rotates, negative-charged toner is electrostatically attracted from developing unit 7 at the developing station as a result, electrostatic latent image is changed to a visible image or a toner image on the surface, of the drum 5.
Moreover, copy paper is sent out through a pair of feed rollers or resister rollers 10 from a selected cassette stored in paper feed unit 8 with proper timing to keep the front edge of the toner image in line on the drum 5 with the front edge of the copy paper, and the toner on the surface of the drum 5 is then transferred to the copy paper by operation of a transfer electrode 9.
Thereafter, in the case of an electrostatic separation system, for example, the copy paper is separated from photosensitive drum 5 by a separation electrode 11 to which is applied an AC high voltage. The separated copy paper having a toner image is transported to a fixing unit 13, called a roller type fixing unit, generally, the toner image is fixed on the paper when passing the unit. Thereafter, the copy paper is ejected out to a tray (no reference numeral) by rollers 14. There are some instances where, even if a toner image is transferred to a copy paper by transfer electrode 9, a small amount of the toner of the image remains on the surface of the drum 5, therefore, the surface thereof is cleaned up by a cleaning unit having a blade (no reference numeral) of which the lower edge contacts with the surface of the drum, so as to be ready for the next copying process. Copying of an original is thus performed in the above-mentioned cycle.
In a process for controlling the density of a copy image, for example, an original density detector 18 (described hereinafter, see FIG. 8) is provided onto the first mirror unit 41 or the second mirror unit 42 disposed in a space wherein the optical system 4 is movably prepared, and the density of an original to be copied is detected on the copy board by the original density detector when the optical system 4 is preliminarily scanning in the direction of an arrow B (see FIG. 8) by operating the copy start button. In accordance with the detected density the conditions of processes are thereby controlled, such as charging, exposing, developing steps and the like, that is, the substantive copying operation, and thus, a copy paper onto which pattern of the original is copied with a proper density can be obtained.
There normally used an original density detector 18 in wherein a reading spot for reading minute parts of an original is exposed on the original and the detector receives the reflected light therefrom. The original density detector is moved relatively to the original(see FIG. 8, reference numeral 19) so that the reading spot can scan the original. For improving the accuracy of detecting an original density, an important factor is how small is the reading spot area on the original, that is, the area to be detected.
Namely, the smaller the area to be detected is, the more accurate is the density value of the line image density. On the contrary, if the area to be detected is broad, the density value including that of the background surrounding the line image must be detected, so that the detected value becomes lower than the real density value. Therefore the correct line image density value cannot be detected.
Accordingly, such an area to be detected must be as small as the line image density can be correctly detected. However, when a detecting means is constructed so as to make the detection area thereof small, the relationship of the mechanical positions between the detector and an area to be detected i.e., an original surface, must be maintained with a high accuracy, therefore, it becomes difficult to put into practice.
For example, in printing type for news-paper, the most lightface is of the order of 0.10 to 0.15 mm in size. It is therefore difficult to maintain the accuracy of the relationship of the mechanical positions between a detector and an area to be detected if the detection area is made to be a spot of 0.10 mm in width, because a scanning must be made while maintaining the accuracy of the relationship of 0.01 to 0.05 mm. As described above, it has been difficult in a copying machine to detect accurately the density value of a line image original.